1. Field of the Invention
The present invention relates to an electrically powered foldable rearview mirror and, more particularly, to a speed reduction device for an electrically powered foldable rearview mirror for automobiles which is improved so that a thrust reaction on a worm is not transmitted to a motor shaft.
2. Description of Related Prior Art
The electrically powered foldable rearview mirror for automobiles includes an electrically powered motor for folding the mirror which is fixed on a frame of a mirror body, a gear engageably and removably mounted on a shaft which stands on a base and on which the mirror body is mounted, and a speed reduction device located between the motor and the gear.
The gear is provided around the shaft and has a clutch function, being fixed on the shaft during electrical rotation of the mirror and being free of the shaft during forced rotation of the mirror, and the erected or folded state of the mirror body is reached by rotation of the frame about the gear. A worm is directly pressed into an output shaft of the electrically powered motor for folding to transmit the driving force to the gear via the speed reduction gear train following a worm wheel engaging the worm.
The driving force is transmitted from the worm which is mounted on a motor output shaft to the worm wheel and the worm is moved in an axial direction by a reacting rotation force (thrust reaction) due to a transmitted load. Due to the thrust reaction on the output shaft, an end of the output shaft is decentered and the motor is overloaded abnormally, and abrasion of the motor components is accelerated.
As disclosed by Japanese Published Patent Application No. Hei 6-74500, the electrically powered foldable rearview mirror is provided with a claw coupling (a loose fit coupling) with a clearance between an end of the output shaft of the electrically powered motor for folding the rearview mirror and an end surface of the worm, and the motor output shaft is prevented from the thrust reaction. In the electrically powered foldable rearview mirror described above, a penetrating portion which has a width between flat surfaces defined by a pair of chamfered portions is provided at the end of the motor output shaft and is inserted to engage a grooved portion which is provided on a motor adjacent end of a worm shaft so as to transmit the driving force.
In the system described above, the chamfered portions of the motor output shaft end contact a wall of the worm shaft end and the driving force is transmitted, thus rotating the worm. Furthermore the claw coupling is formed so that the chamfered portions of the motor output shaft are inserted in the groove formed on the end of the worm so that the end surface and the root of the chamfered portions do not contact the top and the bottom of the groove so that the thrust reaction on the worm is not transmitted to the motor. Since the loose fit coupling forming the claw coupling of the motor shaft and the worm is small, the worm repeats normal or reverse rotation every time the mirror body rotates into the erected state or the folded state, and the driving force cannot be sufficiently transmitted due to an abrasion caused on contact of the plane corner of the motor output shaft side and the groove wall of the worm shaft side.